Centrifugal separation



March 19,1968 G. c. ROBINSON, JR, ET 3,374,039

CENTRIFUGAL SEPARATION Filed Feb. 26, 19 65 INVENTOR5 WALTHER SCHMIDT OGLE R. SINGLETON, JR.

GROVER C. ROBINSON, J R

14m, 64m yaw,

ATTORNEYS United States Patent 3,374,089 CENTRIFUGAL SEPARATION Grover C. Robinson, Jr., and Ogle R. Singleton, Jr., Richmond, and Walther Schmidt, Henrico County, Va., as-

signors to Reynolds Metals Company, Richmond, Va.,

a corporation of Delaware Filed Feb. 26, 1965, Ser. No. 435,596 7 Claims. (Cl. 75-68) ABSTRACT OF THE DISCLOSURE Aluminum-silicon alloys containing at least 50 Wt. percent of aluminum and 20 wt. percent of silicon are beneficiated by melting the alloy, cooling the melt in a quiescent state to cause formation of a skeletal crystalline solid phase having entrained therein residual liquid phase, and centrifugally separating the liquid phase from the solid phase.

This invention relates to method and apparatus for the separation of a metallic liquid phase from a partially solidified melt. More particularly, the invention concerns a novel method for the segregation of hypereutectic alloys containing at least 50% aluminum and at least 20% silicon into solid and liquid phases at temperatures above the eutectic temperature.

It is well known that the properties of metallic alloys are influenced by the nature and proportions of the components. The components or their intermetallic compounds may be soluble mutually, wholly, or only partially up to certain limits of saturation, depending upon temperature. These solubility relations have great influence upon the progression of freezing of the alloy from the molten state and upon the structure of the resulting solids. In the case of single pure metals, or of pure eutectics or other intermetallic compounds which act essentially as a unit substance, freezing takes place at constant freezing temperature to an aggregate of homogeneous crystalline structure. With other relations of solubility, solidification proceeds selectively, being spread over a range of temperature.

The present invention is concerned particularly with the behavior and treatment of hypereutectic aluminum alloys, especially those containing, by weight, at least 50% aluminum and at least 20% silicon, the remainder comprising various amounts of iron or titanium or both. Thus, the amount of iron can range up to about 15%, and the amount of titanium up to about 10%; and the alloy also may contain about 0.14% carbon. Alloys of this type can be segregated into solid and liquid phases at temperatures above the eutectic temperature. The invention especially includes the separation, including purification, of thermally smelted alloys which usually contain (in addition to Al, Si, Fe and Ti) calcium, phosphorus, and contaminations such as carbides, oxides, and oxycarbides. The predominant carbide is SiC, which may run as high as 10% in any particular alloy.

Generally, on cooling a molten alloy of this type, soluble non-metallics and TiSi start to crystallize out first. The crystallization continues in the general pattern set by the known Fe-Al-Si phase diagram. As an example,

within the particularly useful range of at least 50% Al,

at least about 20% Si, 0.5 to 10% Fe, 0.1 to 5% Ti and less than 2% carbon, the order of crystallization is as follows:

1675" F.: SiC+TiSi (little Si) 1400 F.: TiSi +Si (little SiC and little FeAlQSi 1130 1 Si+FeAl Si (little SiC and little TiSi If Ca is present, CaSi will be mainly in the crystals obtained at 1130 F. Such alloys have a eutectic which solidifies at about 578 C. and has a composition: 11.7% Si, 0.8% Fe, 0.08% Ti, about 0.02% Ca, remainder aluminum. On cooling from the molten state and separating at a practical temperature above the eutectic temperature, e.g., 590 to 650 C., hypereutectic liquid phases are obtained. The higher the temperature, the more the Si, Fe and Ti stay in solution. A typical composition separated as liquid at 610 C. is 13.5%-l4% Si, 1.2% Fe,

and 0.1% Ti. Additionally, the amount of iron may be decreased, if desired, by adding copper (up to about 4%) or magnesium (up to about 1%), or both, prior to crystallization, to provide a lower melting eutectic composition.

For an alloy of the character described, freezing from the melt results in segregation, with progressive enrichmerit of the molten composition toward the composition which has the lowest freezing point. The first portion or portions to freeze will have an opportunity to solidify in crystal form and with freedom for movement in the remaining liquid. The remaining liquid is finally forced to the composition of lowest and final freezing temperature, the so-called eutectic temperature and composition, constant for a particular alloy, whereupon it will solidify and occupy such space as may remain between the particles making up the solid phase frozen during the range of selective freezing. If the molten alloy is cooled to an intermediate temperature, the hypereutectic segregated composition will remain liquid within the spaces in the solid phase.

Methods have been suggested in the prior art whereby a molten aluminum alloy is allowed to cool to form a crystal mass of non-eutectic material, which is separated from a Inenstruum of liquid phase which may be eutectic or most commonly hypereutectic, preferably near to eutectic alloy, by centrifugation e.g. in a basket centrifuge, but these methods have proven cumbersome and inefiicient. Other attempts have been reported using a centrifugal force, resulting in an acceleration at the periphery of 1,000 to 2,500 g., the aim being to move the lighter solids, e.g., silicon, towards the center for continuous removal. However, aside from the mechanical difiiculties of operating hot mixtures with such high forces, the separation Was incomplete and required a subsequent filtration.

In contrast, the present invention recognizes essential advantages in not removing solids at all, by producing a skeletal crystalline solidphase, having relatively high physical strength, and applying a centrifugal force below that which could destroy the structural integrity of the solid phase. In addition, the skeletal structure is used as a filter in which undesired solid contaminants are trapped. The invention accordingly provides means for obtaining such a skeletal structure having a compressive strength at temperatures around 1150 F., of 10-40 lbs/sq. in., while the centrifugal force is limited to a force which will not change the structure and shape of the solid phase.

Therefore, in accordance with the present invention, there is provided a novel method for the: separation of an alloy into solid and liquid phases involving the formation under controlled conditions of a cake of solid phase alloy, which is structurally rigid and capable of retaining its structural pores and channels when subjected to moderate centrifugal forces. tained by the method of the invention comprises a rigid skeletal matrix having distributed throughout its inner portion, a multiplicity of interconnecting cells or pores, which communicate with each other, permitting liquid phase to collect and to be separated by centrifugal or comparable means. Factors which influence the formation The solid phase alloy obof strong skeletal crystalline phase and which are controlled in accordance with the invention include (a) the composition of the alloy, and the ratio of liquid to solid at a selected temperature and (b) the rate of cooling.

The novel method and apparatus of the invention are adapted for the separation of liquid and solid phases of metallic alloys in general, but will be illustrated with reference to the treatment of the aforementioned alloy of aluminum and silicon, for which the invention is especially adapted. The principles of centrifugal separation in accordance with the invention will be described as illustrative, but it is to be understood that the invention is not to be considered as limited thereto.

The method of the invention comprises the steps of introducing a raw molten alloy into the bowl of a centrifuge, allowing the substantially undisturbed (quiescent) mass of molten alloy to decrease in temperature by regulation of the temperature gradient of cooling to form in part a rigid skeletal solid phase containing distributed throughout its inner portion a multiplicity of interconnecting passages, and a liquid phase, said liquid phase being distributed in said passages, said spongy solid phase having strength sufficient to withstand the application of centrifugal forces without injury thereto, and then centrifuging to separate said liquid phase from said solid phase. The solid phase is obtained in the form of a cake, comprising the selectively crystallized portion of the mixture.

The centrifuge bowl arrangement which is adapted to achieve the objective of controlled formation of a crystalline cake, in accordance with the invention, is of special design. Two embodiments of this apparatus are illustrated in the accompanying drawings, in which:

FIGURE 1 is a schematic cross-sectional view of a centrifugal separator having a continuous outer shell and a removable inner cone;

FIGURE 2a is a schematic cross-sectional view of a second form of centrifugal bowl having a removable heatretaining wall; and

FIGURE 2b shows the apparatus of FIG. 2a, with the removable wall in a raised position.

In the embodiment shown in FIG. 1, the centrifugal bowl 11 is mounted upon a drive shaft 12, driven by power means not shown. Molten alloy is supplied to the centrifuge bowl from a supply source shown generally at 13, which may be a melting furnace or a reduction furnace. A charge of molten alloy in the bowl 11 is allowed to cool to the desired temperature under controlled conditions, forming cake 16, which collects around a conical support 17 of refractory covered steel or similar material which is located in the center of the bowl and in alignment with shaft 12. The conical support rests upon a circular plate 18, kept in alignment with the shaft 12 by a central recess 19 into which the upper end of the shaft extends. The conical support 17 has attached at its upper end a shaft 20 by means of which the support and cake 16 can be lifted out of the centrifuge bowl when the centrifuging operation is completed. Bowl It]; is provided at its upper edge with an annlar flange 22, over which the liquid flows into annular collecting trough 21, which is provided with a splash baffle wall 23.

In the operation of the form shown in FIG. 1, the molten alloy is allowed to cool undisturbed until a rigid and strong skeletal structure has formed, the passages of which are permeated with molten liquid phase. The bowl is then subjected to rotation at a speed providing a force sutficient to expel the liquid phase from the interior of the cake and to the interior wall of the bowl, whereby the liquid phase rises along the inner surfaces of the bowl, and overflows the upper edge of the bowl into the circular trough 21 surrounding the bowl. When separation of solid and liquid phases is complete, the solid cake and supporting cone 17 are lifted out of the apparatus.

In the embodiment of the apparatus shown in FIG. 2a, the centrifuge bowl comprises a fiat bed 30, mounted upon a driving shaft 31, Which extends through the floor of the bed to provide a projection or protuberance 32, which serves to anchor the solid cake when formed. The inwardly tapering wall 33 is lifted from the bed 30 after the solid cake has formed and immediately prior to centrifuging. The wall does not rotate and, when raised, acts in conjunction with splash baffle 37 and trough 34- to permit collection of the liquid phase ejected from the cake rotating with bed 30 (see FIG. 2b). The wall 33 and bed 30 are constructed of heat retardant and resistant materials such as steel lined with refractory ceramic. Molten starting alloy is supplied from a source 36, as shown. The apparatus of FIG. 2 may be fitted, if desired, with a central support such as 17 in FIG. 1, and conversely, the apparatus of FIG. 1 may be used without such support. Although not shown, projection 32 may be extended above the charge 16 and, by means of spider arms, support a screen through which the liquid phase is extracted.

The physical strength of the crystal sponge permits it to suffer moderate, but sufficient centrifugal forces, without the support of an outer retaining wall. This in turn allows less centrifugal force, because the liquid does not require any vertical force component, as in the embodiment of FIG. 1.

The temperature of centrifuging must be selected to insure that the particular alloy exhibits a selectively crystallized solid phase, which in volume corresponds to a solidzliquid ratio of at least 1: 10. If less solid is present, the resulting structure does not sufliciently cake together. A preferred range of operation is 65-90% liquid and 35- 10% solids, although the operation can be conducted at equal parts liquid and solid and even down to about 20% liquid by volume. The recoverable fraction of the available liquid decreases, however, as the absolute amount of liquid decreases. This relationship is not linear, but decreases sharply as the initial proportion of liquid falls below the range from about 65% to about The rigidity of the cake and the efficiency of separation depend also on heat withdrawal within the critical range of temperatures below 1800" F., where most of the crystallization takes place, An alloy tapped from the reduction furnace at about 3000" F. can be quickly cooled down to approximately 2000 F., e.g., by adding an appropriate amount of solid metal, which may, for example, have been obtained by casting a previous tapping into pig moulds. However, below 1800 F., the crystallization must be controlled and must be substantially undisturbed. The withdrawal of heat should be done at a rate resulting in an average temperature drop of not more than approximately F. per minute, preferably less. Uniformity of temperature distribution throughout the alloy should be controlled, allowing a gradient of temperature of the order of i25 F. It is advisable to contain the alloy in preheated and well insulated refractory material and to apply suitable heat, e.g., by hot gases, on the exposed surface. Best results are obtained when, for example, a mass of 1200 lbs alloy is cooled from 1800 F. to 1130 F. over a period of 5-6 hours, e.g., at an average rate of 2 F. per minute. In such case, the alloy is contained in a well insulated bowl and the bowl is covered with a lid, minimizing radiation losses. However, for commercial speed of operations, artificial cooling may be applied, up to an average rate of about 20 F. per minute. It is not advisable to cool much faster, because the network of the skeletal cake becomes finer in size and enmeshes the liquid in smaller enclaves, which either necessitates greater centrifugal force for separation or results in less efficiency.

The choice of rate of solidification also depends on the specific alloy. Larger contents of SiC, Fe, or Ti, e.g., more than 5% of any one, tend to produce a denser network at the same cooling rate compared to alloys having more primary silicon and less Fe or Ti.

Yield and rate of extraction of liquid from the coherent cake structure are influenced unexpectedly by certain variables in the processing steps. For example, in the operation of several size centrifuges up to 1000-lb. capacity it was found that the control characteristic was neither peripheral velocity nor centrifugal g force but was unexpectedly related directly to the rotational speed squared times the outer radius of the skeletal cake times the sum of the internal and external radii of the cake. This may be compared to the theoretical relationship developed by the US. Bureau of Mines in RI:5007, November 1953, that the control characteristic was directly proportional to the rotational speed squared and inversely proportional to the radius of rotation, for centrifuging molten metal by a method in which no solid skeletal structure was provided. This difference is indicative of the fundamental dissimilarities of the present method, which has the benefit of being better adapted for use in commercial sizes and amounts.

It has also been found, in accordance with the invention, that extraction for these aluminous alloys is optimum under solid to liquid volume ratios of 1:10 to 1:2 and is good at a solid to liquid ratio of 1: 1, below which the extractability falls off unexpectedly and sharply to effectively no recovery at 4:1 (80% solid).

There have also been found, in accordance with the invention, effects of particle size upon extractability. Primary Si and intermetallic particle size is increased in any aluminous alloy by controlling and slowing down the quiescent cooling rate, by pretreatment to remove the normal carbon containing contaminants to less than about 2% as carbon, as well as by temperatureof-separation selection to reduce the solid content of separation to as close as the 1:10 ratio as possible. An increase in particle size improves the ease of extraction not directly, but unexpectedly proportionately to the effective diameter of the solid particles to the second power. 7

Final centrifuge speeds employed may range from about 100 to about 2500 r.p.m., in relation to the size of equipment as indicated by the foregoing control characteristic, and in accordance with other considerations set forth herein.

The composition of the liquid phase will be in the range: Si l218%, Fe 0.8-1.8%', Ti 0.10.3%, balance Al. The liquid phase thus obtained has a composition which is similar to those of aluminum-silicon die casting alloys, and can be used for this purpose. The liquid phase also may be used in other types of castings such as those used for making wrought products, especially if the liquid phase is modified by incorporating aluminum having a lower iron content; or it may be used as a beneficiated raw material for production of substantially pure aluminum by known refining processes such as, for example, the aluminum monohalide process.

The described method also offers a means to purify thermally smelted raw alloys from undesired contents of Ca or P. If Ca remains in a casting alloy with more than 0.05%, its effect is harmful for sharp reproduction of patterns. In particular, sharp edges may be rounded off.

If P remains in a casting alloy above 0.005%, it interferes with the well known sodium modification of the Si eutectic. The result is a coarser crystal shape of the silicon within the eutectic and consequently inferior strength characteristics.

It has been found, surprisingly and unexpectedly, that the centrifugal treatment of the invention, whereby .a skeletal structure of silicon and compounds containing silicon is obtained from an aluminum-silicon alloy, as previously described, also achieves an excellent removal of calcium and phosphorus from the liquid phase alloy, these elements being scavenged and remaining in the skeletal solid phase. Thus the liquid phase is suitable for die casting operations as obtained, being low in calcium (about 0.03%) and phosphorus (about 0.001%).

The solid cake is found to function as a cleaning agent for the liquid phase .alloy trapping calcium, e.g., in the form of CaSi between the crystals of silicon as they are formed. It is believed that phosphorus present is also trapped in the silicon crystals in the form of nuclei of aluminum phosphide around which the silicon crystals form.

The performance of the method of the invention is illustrated by the following examples, which are not to be considered limiting.

The following examples concern an alloy containing approximately 30% Si, 4% Fe, 2% Ti, balance essentially aluminum, treated in the removable wall centrifuge of FIG. 2a, to provide a 9.3 cm. exterior radius of the cake at the bottom (and 9 cm. at 4 cm. above the bottom), with a 2.7 cm. radius of the post.

Example I The alloy was treated to be essentially free of carbon impurities. The alloy at 1900 F. was poured into the centrifugal bowl. Cooling time from 1500 F. to 1300" F. was 13 minutes. At 1175 F., the Wall was removed. At 1130 F., the alloy was accelerated to about 2500 r.p.m. in about 30 to 50 seconds and held for about 70 seconds of rotating time prior to deceleration.

63% by weight of the charge was recovered as filtrate product of composition: 14.4% Si, 1.3% Fe, 0.1% Ti, balance essentially aluminum. The original charge was estimated to contain 69.5% liquid at 1130 F. and of that amount of liquid 90.5% was recovered. The residue had and analysis of 59.5% Si, 6.0% Fe, 6.4% Ti, and 25% Al.

Example II The alloy was treated to be essentially free of carbon impurities. The alloy at 1900 F. was poured into the centrifuge bowl. Cooling time from 1700" F. to 1500 F. was four minutes. At 1500 F., the wall was removed. At 1475 F., the alloy was accelerated to about 1000 r.p.m. in about 30 seconds and held at that speed for 1 /2 minutes prior to deceleration.

87% by weight of the charge was recovered as filtrate of composition: 25% Si, 3.5% Fe, 0.39% Ti, balance essentially aluminum. The original charge was estimated to contain 90.5% liquid at 1475 F. and of that amount 95.7 was recovered. The residue had an analysis of 56.7% Si, 1.6% Fe, 14.0% Ti, and 24.8% Al. The central portion of the residue contained 60% Si, 1.4% Fe, 16.7% Ti, 19% Al, andwas more representative of results obtainable either in a larger unit, or if hot gases at 1400 F. were supplied as a blanket over the spinning alloy. Over 80% of the input Ti was trapped in this residue.

The filtrate was collected and reseparated by centrifugal action at 1130 F., yielding a product of 14.4% Si, 1.3% Fe, 0.1% Ti, balance essentially Al, when centrifuged again at 1000 r.p.m. This provided a recovery of 63% of the filtrate charge and 55% of the original charge. Under comparable conditions, one separation at 1130 F. would yield 53% recovery. The residue composition was 48% Si, 8.2% Fe, 1% Ti, and 36.1% Al.

Spinning as above described was accomplished in a room temperature air environment. Yield is increased and control facilitated, however, when the top surface and sides of the alloy mass are provided with an environment of gases above the melting point of the liquid phase (about 1070 F.) and not substantially hotter than the desired separation temperature, thus permitting the metal in the centrifuge to approach thermal equilibrium at the temperature for separation. Without this provision, thermal gradients up to 40 F. occur in a 15-lb. load (and up to F. in a 1000-lb. load), using average cooling rates below 1800 F. of 25 F. per minute and 5 F. per minute, respectively. Non-uniform temperature is undesirable because it tends to cause a non-uniform liquid product. The provision of a temperature-maintaining capability has the added advantage of permitting a greater time leeway for initiating operation of the centrifuge, which is .a practical convenience. Under conditions comparable to Example I, a copious supply of hot gases (1125 F.) was maintained around the cake after the wall was removed. The cake also was insulated from the rotating bed by six layers of Fiberfrax material. The yield increased. to 68% of the charge.

The alloys discussed herein are conveniently prepared by a thermal smelting operation in which metallic ore such as bauxite, alumina or low-silica clay is reduced by carbon, preferably in an electric arc furnace. Some of the carbon can be replaced by substances of a metallic nature, as known in the art. The aluminous metal produced may contain dissolved or entrained non-metallics, especially carbon and carbides, also oxides and nitrides. If the carbon containing impurities were present at a level of 6 to 8%, then one spinning at 1000 rpm. maximum would reduce yield to 46% to 36% from a typical yield of 53% due to increased solids ratio and nucleant effects reducing particle size.

The following additional example illustrates the effect of copper or copper and magnesium additions in depressing iron content of the separated liquid phase.

Example [11 (a) A raw alloy having the following composition by weight was treated in the manner of Example 1:

Si 40.2 Fe 3.6 Cu 0.7 Mn 0.3 Mg 0.2 Ni 0.1 Ti 1.8 A1 Bal.

The recovery was 45.3%, with the beneficiated alloy having the following analysis:

Si 12.7 Fe 0.65 Cu 1.6 Mn 0.12 Mg 0.37 Ni 0.22 Ti 0.1 A1 Bal.

(b) In contrast, a raw alloy containing substantially no copper and magnesium, having the following composition, was treated in like manner:

Si 40.3 Fe 3.3 Cu 0.1 Mn 0.1 Mg .05 N1 .05 Ti 0.10 Al Bal.

Si 14.0 Fe 1.1 Cu .09 Mn .02 Mg 0.1 N1 .05 Ti 0.10 Al Bal.

It is apparent from the results of these comparative tests that the final iron content was considerably lower where copper and magnesium were present prior to contrifuging. The most useful range of additions is between about 1 and 3% copper, and from 0.1 to about 1% magnesium, in order to reduce the iron content of the liquid product below 0.8%. Spin separation then is performed below 8 1130 F. and preferably below 1100 F. Over of the added copper typically is recovered in the liquid phase alloy.

Suitable methods for the removal of non-metallic insolubles prior to centrifuging include (a) high temperature settling, i.e. above 1800 F., and decantation of the upper-purified-portion; (b) high temperature centrifuging, i.e. above 1600 F., to remove purified liquid from a high concentration of non-metallics and some metallic compounds such as TiSi and (c) filtration at temperatures above 1800 F. through alumina, carbon, SiC, anhydrous quartz or other inert beds. On these beds, it has been found that particle size should be controlled such that the minimum size is not much less than effective diameter and can range upwards from this value. About 8" of bed of A1 to 1 /2" metallurgical coke was found to reduce the total carbon content of a melt of raw alloy from about 2% to about 0.2% in a one pass straightthrough filter at about 2600 F.

Although the preceding description has emphasized recovery of the liquid phase, certain useful characteristics also are exhibited by the residual solid phase. Due to removal of the more ductile, lower melting point metal, the cake is easier to crush or grind, and is susceptible to chemical leaching for the purpose of liberating higher melting point brittle phases such as, for example, TiSi Si, or an Al-Fe-Si compound; and the porous cake itself may be used for catalyst support or as a packing for chemical reaction towers.

In addition, the method of the present invention is applicable to the beneficiation of silicon containing impurities which are readily soluble in aluminum. This involves adding aluminum to produce a melt having at least about 25% silicon, .and then operating near the lower limits of solid content and cooling rate to preferentially yield edge-connected crystals of silicon as the solid phase. Such crystals may be separated mechanically.

Another advantage of the invention is that a high melting point purified metal phase can be solidified out at temperatures well below the melting point. For example, an alloy of 40% Al-60% Si (which is completely molten at 2200" F.) can be treated to yield half of its weight as a substantially pure silicon cake, when centrifuged at about 1300 F., although the melting point of silicon is about 2600 F. The coherent silicon matrix can be reinfiltrated by a liquid phase, iron or tin, for example, to produce an artificial composite having useful properties.

While present preferred embodiments of the invention have been illustrated and described, it will be apparent that the invention may be otherwise variously embodied and practiced within the scope of the following claims.

What is claimed is:

1. Method for the beneficiation of an aluminum-silicon alloy containing at least 50% aluminum by weight, at least about 20% silicon, up to about 15% iron and up to about 10% titanium, comprising the steps of: providing said aluminum-silicon alloy in molten condition, cooling the molten alloy in a quiescent state to cause formation of a coherent skeletal crystalline solid phase higher in silicon content than said aluminum-silicon alloy, having therewith a liquid phase lower in silicon content than said aluminum-silicon alloy, the proportion of said solid phase being at least as great as a 1:10 solid-to-liquid volume ratio; and then separating said liquid and solid phases.

2. The method of claim 1, including the step of adding to said molten alloy up to about 4% copper.

3. The method of claim 1, including the step of adding to said molten alloy up to about 1% magnesium.

4. The method of claim 1, including the step of adding to said molten alloy about 1 to 3% copper and about 0.1-1% magnesium.

5. The method of claim 1, including the additional step of recovering silicon from said solid phase.

6. Method for the beneficiation of an aluminum-silicon 9 10 alloy containing at least 50% aluminum by Weight, at solid phase having entrained therein said liquid phase, least about 20% silicon, about 0.5 to 10% iron and about said solid phase retaining its structural integrity 0.1 to 5% titanium, comprising the steps of: when subjected to moderate centrifugal forces; and (a) predetermining the temperature of separation of (c) centrifugally separating said liquid phase from the the alloy upon cooling from the molten state at which 5 solid phase. said alloy is selectively crystallized into a solid phase 7. The method of claim 6, comprising the additional and a liquid phase, at a solid-to-liquid volume ratio step of removing insoluble contaminants from said molten in the range of about 1:10 to about 1:2, said liquid alloy prior to centrifuging.

phase having a lower silicon content than said alloy and the solid phase having correspondingly higher 10 References Clted sill-con content; UNITED STATES PATENTS (b) cooling the molten alloy in a quiescent state to 2,198,673 4/1940 Lovenstein 7563 said temperature of separation at a rate representing 2,464,610 3/ 1949 Regner et a1. 75-68 an average drop in temperature, within the range T below 1800 F., of not more than about 50 F. per 15 DAVLD RECK Prlmary Examiner minute, to cause formation of a skeletal crystal line N. P. BULLOCH, H. W. TARRING, AssistantExaminer-v. 

